In the art of electrowinning aluminum by electrolysis of alumina dissolved in molten cryolite, considerable efforts have been made to provide dimensionally stable materials for cell components which are in contact with the liquid contents of the cell. Such components include the electrodes as well as lining materials and elements which are immersed in the liquid aluminum to restrict hath movements.
Among the materials proposed for use under the severe corrosion conditions in a molten salt electrolysis cell are primarily the refractory oxides; the Refractory Hard Metal (RHM) borides and cermets containing either of them together with an intimately mixed metallic phase for applications where high electrical conductivity is essential.
Refractory ceramic and cermet materials are known from numerous publications. These materials are used in a wide variety of applications, and their specific composition, structure and other physical and chemical properties may be adapted to the specific intended use.
Materials which were proposed for their use as anodes in molten salt aluminum electrowinning cells are mainly based on oxides of e.g. iron, cobalt, nickel, tin and other metals, which oxides may be provided with enhanced electronic conductivity by doping, non-stoichiometry and so forth. Cathodic materials are mainly based on titanium diboride and similar RHM boride compounds. For example, EP-A-0 115 689 discloses reaction sintered oxide-boride ceramic bodies and EP-A-0 116 809 discloses a porous oxide-boride cermet which is infiltrated with molten aluminum. Oxide-boride ceramics have however not proven to be commercially acceptable as cathode materials in molten salt electrolysis.
FR-A-2 375 349 has described bipolar electrodes with an anode made of a boride/silicide/carbon composite and a cathodic section of various borides including titanium diboride and yttrium boride.
A completely new concept for a dimensionally stable inert anode for an aluminum cell and its manufacture was described in EP-A-0 114 085 wherein such anodes are produced by depositing in-situ a fluorine-containing oxycompound of cerium (referred to as "cerium oxyfluoride") on an anode substrate during electrolysis, with a cerium compound dissolved in the melt and maintained at a suitable concentration. This anode coating is maintained dimensionally stable as long as a sufficient concentration of the cerium-containing compounds is maintained in the melt.
In EP-A-0 094 353 it has also been proposed to use materials in a molten salt aluminum electrowinning cell which are composed of a refractory ceramic coated with TiB.sub.2 and wherein the TiB.sub.2 coating is maintained by addition of titanium and boron to the liquid aluminum.
A co-pending patent application Ser. No. 322,850, which was simultaneously filed with the present application discloses a new substrate material for the above described cerium oxyfluoride anode coating, this new substrate material being a cermet having a ceramic phase basically comprising a mixture of cerium oxide(s) and alumina and a metallic phase comprising an alloy of cerium and aluminum.